In almost as many flights, good thing they're cheapies.

As some of you may remember I electrified my Chipmunk. Everything seemed to be going OK, but now it's flying time and it's cooked two motors in short order. It's got a big intake under the spinner and the area aorund the esc is releived, allowing blow-by air, and behind that, I've put in a set of 'exhaust pipes', oversized carbon tubes to look like engine exhausts. I thought I had adequate cooling to the motor, but it seems I maybe I don't.

Upon the last one fizzing out I checked the temp of the windings and they were around 92˚C (197˚F). This seems to be the point where the enamel coatings on the windings melt and short.

Strangely, the pack was cool to the touch, and the ESC is out in the breeze, and was also virtually 'room temp'.

This is the first front mount I've done in a while and frankly I'm baffled that I cooked two motors, given the great big mouth the chip has!

Maybe some models just dont like being airborne.

With the cowl opening below the motor, I've had the airflow bypass the motor.  The cure was to put in a baffle to force the air up and across the motor.  Just make sure there is a path for the air to get out after it crosses the motor.  I put holes in the firewall above the motor and vent it into the battery compartment, then out the exhaust holes.  Also, some motors just run hot (less efficient). I tried one motor on a profile that ran extreemly hot.  Switched motors (different brand) and it finished the flight with the motor just warm to the touch.

Use heat sink, it is very effective way to cool it down (second best after usage larger motor).

rotor does not need cooling, it does not heat, so outrunner or not, still on stator, it is fixed, so does not make problems

it could be you had overheted also ESC, I do not know how could ESC see overheated motor without a temperature sensor

for WIRES? coil heats direcly stator (resistive loses) and loses in iron are directly in stator, so why long way? it cools direcly the part which heas most, I have it always hot after landing, its temperature is certainly hiher then temperature of rotor

Greg,
You didn't indicate what motor of prop you are using or the normal static amp draw. You might be on the edge of what the particular motor can handle as far as prop load which the higher temp pushed it over. Check the amp draw, my guess for that size ship would be around 24 amp. All mine are front mount, electric plastic spinner no special open nose. They run <110F, you can grab the motor at the end of the flight. 147F you can't touch so something is loading the motor causing it's temp to be that hot. Try reducing the prop diameter 1" and see if the amps drop, if not check for something rubbing inside the cowl. Keep us posted.

Best,       DennisT

Thanks for the input guys, the setup was a direct transfer from a similar sized profile, that's had well over 100 flights without issue, which is why I'm pointing the finger at cooling.

The setup was a turnigy SK3 35-36 1050kw, 4S 3000mah pack, and a 45 amp speed controller. Prop was an 11x5.5.
 
In hindsight I had the idea a turbo backplate may not be pushing air into the motor, but actually drawing air into the spinner and ejecting it out the prop blade holes, (centrifugal pump basically). I think I may have short circuited the airflow, going directly from nose scoop to prop holes, and bypassing the motor altogether.

In all honesty, I was never really happy with the chip, it has some issues. My partner thinks it's cursed, and has suggested I build a new one and leave the chip is wall art. I'm half inclined to agree!

Dennis, yeah, having a look at Ecalc, considering I obviously have cooling issues, it'll be happier on a 10x6 than an 11 x5.5 for the same aircraft speed.

It might be a good test to see just how much of a difference lower RPM, and higher pitch makes to the overall 'crispness' of the model.

With a Renecle timer governor, it pays to have some extra voltage up your sleeve, so I tend to do most of my calcs based on "3.5 cells. 1000kv works at around 10000rpm.

...and like I say, that exact setup has been working happily in a profile for the last two years.

I find it pays to have some when people irritate me.